The various machinery used in the ferrous metal, papermaking, wind power generation, mining, and other various fields, in automobiles, and in rolling stock all use various types of ball bearings. These ball bearings are used under harsh conditions under high loads, so cracks are liable to locally form.
Ball bearings are standardized in, for example, JIS-B1518. In this standard, in the case of a radial ball bearing, it is recommended that a cross-sectional profile line of a race at an inner race side (outer race side) (component of ball bearing defining and forming path over which rolling balls move) have a radius of curvature of a groove of 52% or less (53% or less) of a diameter of the rolling balls. On the other hand, in the case of a thrust ball bearing, it is recommended that both the cross-sectional profile lines of the upper race side and lower race side of races have a radius of curvature of the groove of 54% or less of the diameter of the rolling balls. However, in the ball bearings according to this standard, the cross-sectional profile line of the races are arcs having a single radius of curvature, so the contact area of the rolling balls and races at the time of a load cannot be sufficiently secured. For this reason, in particular at the time of a high load, a high pressure acts on the parts of the races contacting the rolling balls and those parts are liable to crack.
Therefore, in recent years, ball bearings reducing the pressure acting on the races and suppressing cracking (Japanese Patent Publication No. 2009-174691A: PLT 1) and rolling machine elements strikingly improving the load capacity and suppressing cracking (Japanese Patent No. 3608163: PLT 2) have been disclosed.
PLT 1 discloses a race having a cross-sectional profile line comprised of an arc part of the same radius as the radius of the rolling balls and tangential parts formed as extensions of the arc part. According to PLT 1, by selecting the circumferential length of the arc part suitably for the load, it is considered possible to suppress differential slip and sufficiently secure the contact area of the rolling balls and arc part to suppress the surface pressure applied to the race.
Further, PLT 2 discloses a race having a cross-sectional profile line comprised of a composite arc made of a plurality of arcs with different radii of curvature smoothly connected, in which plurality or arcs, the radius of curvature at the center in the width direction is relatively small and the radii of curvature at the two sides in the width direction are relatively large. According to PLT 2, by adopting this configuration, it is possible to reduce the pressure on the race. It is considered that this reduction of pressure can advantageously act against the rolling fatigue of the surface of the race and against overriding of the shoulders and can reduce differential slip.
Note that, in addition to PLTs 1 and 2, the technique of making the shape of the path of the rolling balls a secondary curve etc. (Japanese Patent Publication No. 40-7608B: PLT 3) and the technique of changing the radius of curvature of the curved path of the rolling balls from the groove bottom to the shoulder parts (Japanese Patent Publication No. 53-139047A: PLT 4) are disclosed.